Grease composition for ball joint

ABSTRACT

An object is to provide a grease composition for a ball joint capable of reducing the starting torque, reducing the operating torque, and improving the low temperature property as compared with the related art. The grease composition for a ball joint according to the present invention contains a silicone oil as a base oil and an additive, and contains an amino-modified silicone and a modified polyolefin as the additive. The silicone oil preferably includes at least one of dimethyl silicone oil and phenyl-modified silicone oil.

TECHNICAL FIELD

The present invention relates to a grease composition for a ball joint.

BACKGROUND ART

Conventionally, for example, a grease using mineral oil or synthetic hydrocarbon oil has been used for a ball joint such as a suspension or a steering of an automobile (refer to Patent Literature 1).

In order to maintain the lubricity of the ball joint, the viscosity of the grease is increased to improve the adhesiveness of the grease, for example, by using a base oil having a relatively high viscosity, or improving a thickener.

CITATION LIST Patent Literature

Patent Literature 1: JP 2000-230188 A

SUMMARY OF INVENTION Technical Problem

However, the conventional grease for a ball joint is problematic in that the starting torque of the ball joint becomes high. The invention described in Patent Literature 1 does not refer to the starting torque of the ball joint.

In addition, as described above, setting the viscosity to be high for maintaining the lubricity of the grease for the ball joint hardly maintains the operating torque of the ball joint at a low level under a low temperature.

Therefore, the present invention has been made in view of such a problem, and an object thereof is to provide a grease composition for a ball joint, the composition capable of reducing the starting torque, reducing the operating torque, and improving the low temperature property, as compared with the related art.

Solution to Problem

The grease composition for a ball joint according to the present invention contains a silicone oil as a base oil and an additive, and contains an amino-modified silicone and a modified polyolefin as the additive.

With this configuration, the silicone oil as the base oil can maintain the fluidity of the grease composition at a low temperature. In addition, the amino-modified silicone and the modified polyolefin as additives are bonded to form a strong oil film, and therefore when the grease composition is used as a lubricant for a ball joint, oil film breakage can be prevented during the stopping of the ball joint. This can improve the operability of the ball joint at a low temperature while reducing the static frictional force of the grease composition.

In the grease composition for a ball joint according to an aspect of the present invention, it is preferable to include at least one of dimethyl silicone oil and phenyl-modified silicone oil as the silicone oil. With this configuration, the dimethyl silicone oil and the phenyl-modified silicone oil have a higher viscosity index than other base oils, and have characteristics of hardly changing the viscosity irrespective of temperature change, thus allowing maintaining the fluidity of the grease.

In the grease composition for a ball joint according to an aspect of the present invention, the amino-modified silicone is preferably represented by the following general formula (1) or (2).

-   -   Organic group is —RNH₂, R represents an alkyl group, and n         represents number of repetitions.

-   -   Organic: group is selected from any

-   -   R and R′ represent an alkyl group, and m and n represent number         of repetitions.

With this configuration, these amino-modified silicones are effectively bonded to the modified polyolefin, and the modified polyolefin can be uniformly dispersed in the base oil.

In the grease composition for a ball joint according to an aspect of the present invention, it is preferable to include at least one of oxidized polyethylene, oxidized polypropylene, acid-modified polyethylene, and acid-modified polypropylene as the modified polyolefin. With this configuration, these modified polyolefins can be effectively bonded to the amino-modified silicone, allowing satisfactorily forming an oil film.

In the grease composition for a ball joint according to an aspect of the present invention, the content of the amino-modified silicone is preferably more than 0 parts by mass and 30 parts by mass or less. In addition, the content of the modified polyolefin is preferably more than 0.1 parts by mass and 20 parts by mass or less.

In the grease composition for a ball joint according to an aspect of the present invention, the content of the amino-modified silicone with respect to the modified polyolefin is preferably in the range of the following formula (3).

0.5≤(mass of amino-modified silicone/mass of modified polyolefin)≤30.0  (3)

With these configurations, an oil film can be effectively formed by effectively acting the amino-modified silicone and the modified polyolefin while uniformly dispersing the modified polyolefin in the base oil.

In the grease composition for a ball joint according to an aspect of the present invention, the modified polyolefin preferably has an acid value of more than 0.1 (mgKOH/g) and 100.0 (mgKOH/g) or less. With this configuration, the amino-modified silicone can be effectively bonded thereto, allowing satisfactorily forming an oil film.

In the grease composition for a ball joint according to an aspect of the present invention, the static friction coefficient of the grease composition is preferably 0.050 or less. With this configuration, the static frictional force can be effectively reduced.

Advantageous Effects of Invention

The grease composition for a ball joint according to the present invention can be preferably used as a lubricant for a ball joint used in, for example, an automobiles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view for explaining a test method of a static friction coefficient.

FIG. 2 is a cross-sectional view of a ball joint used in an experiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment (hereinafter, abbreviated as “embodiment”) according to the present invention will be described in detail. The present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention.

As a result of intensive investigations for a grease composition used for lubricating a ball joint, the present inventors have improved the static frictional force and the operability at a low temperature by using a silicone oil as a base oil and using an amino-modified silicone and a modified polyolefin as additives.

(Grease Composition for Ball Joint)

The grease composition for a ball joint according to the present invention contains a silicone oil as a base oil and contains an amino-modified silicone and a modified polyolefin as an additive. A thickener or the like conventionally included as a grease composition is appropriately added.

The silicone oil as the base oil is not limited, and it is preferable to include at least one of dimethyl silicone oil and phenyl-modified silicone oil.

The kinematic viscosity of the base oil is preferably 100000 mm²/s or less at 25° C. In addition, the kinematic viscosity of the base oil is more preferably 10000 mm²/s or less at 25° C., and the lower limit thereof is not particularly limited, and for example, the kinematic viscosity of the base oil can be lowered to about 5 mm²/s at 25° C.

In addition, the amino-modified silicone as an additive is preferably represented by the following general formula (1) or (2).

-   -   Organic group is —RNH₂, R represents an alkyl group, and n         represents number of repetitions.

-   -   Organic: group is selected from any

-   -   R and R′ represent an alkyl group, and m and n represent number         of repetitions.

Herein, the numerical value is not limited, and m is several tens or more, specifically 50 or more, and is about 80 to 750 in examples described later. In addition, n is about 100 or less, specifically 20 or less, and is about 1 to 10 in examples described later.

In addition, the kinematic viscosity of the amino-modified silicone is preferably 15000 mm²/s or less at 25° C. This kinematic viscosity can be adjusted by the number of repetitions of the chemical formula described in the above general formula.

The modified polyolefin of the additive preferably includes at least one of oxidized polyethylene, oxidized polypropylene, acid-modified polyethylene, and acid-modified polypropylene. In addition, the acid value of the modified polyolefin is preferably more than 1 (mgKOH/g) and 100.0 (mgKOH/g) or less. In addition, the acid value of the modified polyolefin is more preferably 5 (mgKOH/g) or more and 80 (mgKOH/g) or less, and still more preferably 15 (mgKOH/g) or more and 60 (mgKOH/g) or less.

The dimethyl silicone oil and the phenyl-modified silicone oil as base oils have a higher viscosity index than other base oils, and have characteristics of hardly changing the viscosity irrespective of temperature change. Therefore, the fluidity of the grease can be maintained by using at least one of dimethyl silicone oil and phenyl-modified silicone oil as the base oil.

In addition, the amino-modified silicone as an additive is bonded to the modified polyolefin as an additive, allowing uniformly dispersing the modified polyolefin in the base oil. In addition, the site where the amino-modified silicone and the modified polyolefin are bonded has polarity and is easily adsorbed to the metal surface. Therefore, the mixture of the amino-modified silicone and the modified polyolefin forms a strong oil film on the metal surface of the ball joint, allowing preventing oil film breakage during the stopping of the ball joint. Therefore, the grease composition for a ball joint according to the present embodiment can reduce the static frictional force and improve the operability of the ball joint at a low temperature.

In the grease composition for a ball joint, the content of the amino-modified silicone is preferably more than 0 parts by mass, more preferably 3 parts by mass or more, and still more preferably 4 parts by mass or more. In addition, the content of the amino-modified silicone is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and still more preferably 20 parts by mass or less.

The content of the modified polyolefin is preferably more than 0.1 parts by mass, more preferably 0.5 parts by mass or more, and still more preferably 1.0 parts by mass or more. In addition, the content of the modified polyolefin is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and still more preferably 10 parts by mass or less.

In addition, the content of the amino-modified silicone with respect to the modified polyolefin is preferably in the range of the following formula (3).

0.5≤(mass of amino-modified silicone/mass of modified polyolefin)≤30.0  (3)

Mass of amino-modified silicone/mass of modified polyolefin is preferably 0.5 or more, more preferably 0.7 or more, still more preferably 0.9 or more, and furthermore preferably 1.0 or more. In addition, mass of amino-modified silicone/mass of modified polyolefin is preferably 30.0 or less, more preferably 10.0 or less, still more preferably 5.0 or less, and furthermore preferably 3.0 or less. Defining the contents of the amino-modified silicone and the modified polyolefin can effectively act the amino-modified silicone and the modified polyolefin to effectively form an oil film while uniformly dispersing the modified polyolefin in the base oil.

In the present embodiment, in addition to dimethyl silicone oil and phenyl-modified silicone oil, different base oils can be mixed and used. However, when a base oil other than dimethyl silicone oil and phenyl-modified silicone oil is combined, it is preferable that half or more of the base oil in the grease composition is composed of at least one of dimethyl silicone oil and phenyl-modified silicone oil.

In the present embodiment, it is preferable to adjust the total content of the additives within a predetermined range. Too large content of the additive easily causes oil separation, and therefore specifically, the content of the additive is preferably 50 parts by mass or less. Herein, 100 parts by mass means a ratio with respect to 100 parts by mass of the sum of the base oil and the additive. The same applies hereinafter.

The static friction coefficient in the present embodiment of the grease composition for a ball joint configured as described above is preferably 0.050 or less, more preferably 0.045 or less, and still more preferably 0.040 or less. This effectively reduces the static frictional force. The static friction coefficient in the present embodiment depends on the experimental conditions of the static friction coefficient of examples.

The static friction coefficient is determined by dividing the tensile force (sliding resistance) of the steel sheet by a load wherein a test piece to which a predetermined load is applied and the steel sheet coated with the grease composition are slid (refer to examples).

The thickener included in the grease composition of the present embodiment is not particularly limited, and can be selected from, for example, at least one of lithium soap, calcium soap, sodium soap, aluminum soap, aluminum complex soap, lithium complex soap, calcium complex soap, aluminum complex soap, urea compound, organic bentonite, polytetrafluoroethylene, silica gel, and sodium terephthalate. In addition, for example, a fatty acid amide can be used. The lithium soap is a product from saponification reaction of a fatty acid or a derivative thereof and lithium hydroxide. The fatty acid to be used is at least one selected from the group consisting of saturated or unsaturated fatty acids having 2 to 22 carbon atoms and derivatives thereof. In addition, “soaps” obtained by reacting the fatty acid or a derivative thereof with lithium hydroxide are commercially available, and these soaps can also be used.

In addition, for example, an antioxidant, a rust inhibitor, a metal corrosion inhibitor, an oily agent, an antiwear agent, an extreme pressure agent, and a solid lubricant can be added as necessary. The content of these additives is within a range of about 0.01 parts by weight to 20 parts by weight.

The antioxidant can be selected from, for example, hindered phenol, alkylated diphenylamine, hindered amine light stabilizers, and phenyl-α-naphthylamine. According to experimental results described later, an antioxidant is added in an amount of about 1 part by weight. Thereby, good dust cover compatibility can be obtained.

The dust cover is a very important member for preventing leakage of grease and mixing of dust into the ball joint, and the dust cover is composed of, for example, chloroprene rubber and has a property of contracting by heating. Shrinkage of the dust cover causes leakage of grease and mixing of dust, and therefore it is necessary to prevent shrinkage of the dust cover.

Herein, a silicone-based grease does not penetrate into the dust cover, and therefore an antioxidant is blended in the silicone-based grease in the present embodiment in order to prevent shrinkage of the dust cover. As a result, the antioxidant acts on the dust cover, and shrinkage of the dust cover can be effectively suppressed.

Typically, the silicone-based grease has high oxidation stability, and therefore an antioxidant is unnecessary unless the silicone-based grease is used at a high temperature (for example, 150° C. or more); however, in the present embodiment, the antioxidant is appropriately added to silicone-based grease in order to exhibit antioxidation performance to the dust cover instead of antioxidation of the grease.

A mineral oil-based grease exhibits an action of causing oil to permeate into the dust cover and swelling, and thus the shrinkage ratio is reduced.

In addition, the rust inhibitor can be selected from carboxylic acids such as stearic acid, dicarboxylic acids, metal soaps, carboxylic acid amine salts, metal salts of heavy sulfonic acids, or carboxylic acid partial esters of polyhydric alcohols. The metal corrosion inhibitor can be selected from benzotriazole or benzimidazole. The oily agent can be selected from amines such as laurylamine, higher alcohols such as myristyl alcohol, higher fatty acids such as palmitic acid, fatty acid esters such as methyl stearate, or amides such as oleylamide. The antiwear agent can be selected from zinc-based, sulfur-based, phosphorus-based, amine-based, or ester-based. The extreme pressure agent can be selected from zinc dialkyldithiophosphate, molybdenum dialkyldithiophosphate, olefin sulfide, oil sulfide, methyltrichlorostearate, chlorinated naphthalene, iodinated benzyl, fluoroalkylpolysiloxane, or lead naphthenate. In addition, the solid lubricant can be selected from, for example, graphite, fluorographite, polytetrafluoroethylene, melamine cyanurate, molybdenum disulfide, and antimony sulfide.

In addition, the consistency of the grease composition can be about 200 to 500. The method of measuring consistency will be described in the section of examples described later.

In the present embodiment, modified silicones such as carboxylic acid-modified silicone can be appropriately added as an oily agent for reducing the torque.

The ball joint using the grease composition of the present embodiment can be used for, for example, a movable part of a suspension or a steering wheel of an automobile (for example, lower arm or tie-rod end). This can improve steering stability and steering performance of the automobile.

EXAMPLES

Hereinafter, the present invention will be described in detail with reference to examples performed to clarify the effects of the present invention. The present invention is not limited by the following examples at all.

In each of examples and each of comparative examples, a grease composition for a ball joint was produced by changing the type and composition of the additive, and the consistency and the static friction coefficient of the grease composition were measured. Dimethyl silicone oil (KF96-1000cs manufactured by Shin-Etsu Chemical Co., Ltd.) was used as the base oil. Lithium soap was used as a thickener. Furthermore, the starting torque at −40° C. of the grease composition of each of examples and each of comparative examples was measured by using the low temperature torque test method of JIS K2220. The results are shown in Table 1.

TABLE 1 Lubricating grease composition Acid Example value Example Example Example Example Example Example Example Example (mg/KOH) 1 2 3 4 5 6 7 8 Base oil Base grease (KF96-1000cs) 85 80 90 85 94 70 85 85 Additive Amino-modified silicone oil-1 10 15 5 10 Amino-modified silicone oil-2 10 4 20 Amino-modified silicone oil-3 10 Acid-modified polyolefin-1 12 5 5 5 Acid-modified polyolefin-2 60 5 2 10 Acid-modified polyolefin-3 45 5 5 Antioxidant-1 Antioxidant-2 Antioxidant-3 Mineral oil Consistency 282 292 277 284 277 302 281 277 Low temperature torque at −40° C. mN · m 120 110 100 120 100 130 110 110 Starting torque mN · m 1587 1771 1649 1577 1853 1790 1987 2155 Static friction coefficient 0.038 0.046 0.044 0.038 0.043 0.04 0.044 0.048 Dust cover compatibility — — X — — — — — Commer- cially Comparative Example available Acid Example Compara- Compara- Compara- product value Example Example Example Example tive Ex- tive Ex- tive Ex- Mineral (mg/KOH) 9 10 11 12 ample 1 ample 2 ample 3 oil type Base oil Base grease (KF96-1000cs) 89 89 89 85 100 95 90 Additive Amino-modified silicone oil-1 5 5 5 5 10 Amino-modified silicone oil-2 Amino-modified silicone oil-3 Acid-modified polyolefin-1 12 5 5 5 5 5 Acid-modified polyolefin-2 60 Acid-modified polyolefin-3 45 Antioxidant-1 1 Antioxidant-2 1 Antioxidant-3 1 Mineral oil 5 Consistency 285 277 282 290 275 283 288 286 Low temperature torque at −40° C. mN · m 110 100 110 120 95 110 120 500 or more Starting torque mN · m — — — 2733 2836 2613 3202 Static friction coefficient 0.045 0.043 0.044 0.046 0.057 0.065 0.053 0.055 Dust cover compatibility ◯ ◯ ◯ ◯ X — — ◯

In addition, “amino-modified silicone-1”, “amino-modified silicone-2”, and “amino-modified silicone-3” in Table 1 correspond to each of the materials shown in Table 2 below. In addition, “acid-modified polyolefin-1”, “acid-modified polyolefin-2”, and “acid-modified polyolefin-3” correspond to oxidized polyethylene, acid-modified polyethylene, acid-modified polypropylene, and oxidized polyethylene, respectively. In addition, “antioxidant-1” corresponds to alkylated diphenylamine, “antioxidant-2” hindered phenol, and “antioxidant-3” hindered amine light stabilizer. In Table 2, the parentheses indicate the viscosities of the amino-modified silicone-1, the amino-modified silicone-2, and the amino-modified silicone-3.

TABLE 2 Organic group —RNH₂

Amino-modified silicone-3 (90)

Amino-modified silicone-1 (1700) Amino-modified silicon-2 (3500)

The consistency was measured by a consistency test method defined in JIS K2220.

In addition, in the present experiment, the static friction coefficient was measured. Experimental conditions for the static friction coefficient are as follows.

<Experimental Conditions for Static Friction Coefficient>

Test piece: SPCC steel plate/φ10 POM ball

Load: 2 kgf

Grease coating film thickness: 0.05 mm

Test temperature: room temperature

Test time: left for 16 hours after application of the grease composition

Sliding speed: 1 mm/sec

Sliding width: 1 mm

Number of sliding times: 1 time

FIG. 1 is a schematic view for explaining a test method of static friction coefficient. In FIG. 1 , reference numeral 3 denotes a POM sphere, and reference numeral 4 denotes a SPCC steel plate. Then, the SPCC steel plate 4 was moved in the A direction while a load was applied in the B direction, and the static friction coefficient was immediately measured. Tensile force moving the SPCC steel plate was measured as sliding resistance.

Based on the measured sliding resistance and the load applied to the POM ball, the friction coefficient was determined by the following formula (4).

Friction coefficient=(sliding resistance/load)  (4)

The friction coefficient obtained from the maximum sliding resistance, which is the maximum value of the sliding resistance, was obtained as the static friction coefficient.

In Examples 1 to 12, the static friction coefficient of the grease composition maintained a low value as compared with Comparative Examples 1 to 3 and the commercially available product. In addition, in Examples 1 to 12, the starting torque at −40° C. was lower than that of the commercially available product. That is, in examples, the static frictional force was reduced, and the operability at a low temperature was improved.

Examples included amino-modified silicone and acid-modified polyolefin as additives. The static friction coefficient of the grease composition in examples was 0.030 or more and 0.050 or less. From examples and comparative examples, it was found that the static friction coefficient of the grease composition is preferably 0.050 or less, more preferably 0.045 or less, and still more preferably 0.040 or less. In addition, the starting torque of the grease composition in Examples 1 to 12 at −40° C. was 130 mN·m or less. From this result, it was found that the starting torque of the grease composition at −40° C. is preferably 130 mN·m or less, more preferably 120 mN·m or less, and still more preferably 110 mN·m or less.

In addition, the dust cover compatibility was measured. In the experiment, the dust cover was immersed in greases of the present example, the comparative example, and the commercially available product, and the volume change after 336 hours while maintaining 120° C. was evaluated.

The volume shrinkage change was 12% or less, indicating ∘, and the volume shrinkage change was more than 12%, indicating x. As shown in Table 1, in Examples 10 to 12 in which an antioxidant was added, good dust cover compatibility was able to be obtained.

In addition, the grease composition for a ball joint having the composition shown in Table 1 was filled between a spherical portion 12 and a surface layer 13 of a ball joint 10 shown in FIG. 2 , and the starting torque was measured. The ball joint 10 includes the spherical portion 12, a receiving member 14 that receives the spherical portion 12, and the surface layer (sheet) 13 that covers the surface of the spherical portion 12. The surface layer 13 is held by the receiving member 14. A shaft portion 11 is integrally provided on the surface of the spherical portion 12. Then, a grease composition 15 is filled between the spherical portion 12 and the surface layer 13. The ball joint of the present embodiment is not limited to the above configuration.

The starting torque was measured by setting the application amount of the grease composition to 0.5 g, the rotation speed to 0.2 rpm, and the rotation angle to 45 degrees. The temperature during measurement was 25° C.

As shown in Table 1, in all of examples, the starting torque was lower than that in comparative examples.

In the present example, it was found that the starting torque can be reduced to 2500 mN·m or less.

In addition, Examples 1 to 12 included 4 parts by mass or more and 20 parts by mass or less of the amino-modified silicone. From examples and comparative examples, it was found that the content of the amino-modified silicone in the grease composition is preferably more than 0 parts by mass, more preferably 3 parts by mass or more, and still more preferably 4 parts by mass or more. In addition, it was found that the content is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and still more preferably 20 parts by mass or less.

In addition, the modified polyolefin was included in an amount of 2 parts by mass or more and 10 parts by mass or less. From examples and comparative examples, it was found that the content of the modified polyolefin in the grease composition is preferably more than 0.1 parts by mass, more preferably 0.5 parts by mass or more, still more preferably 1.0 parts by mass or more, and furthermore preferably 2 parts by mass or more. In addition, it was found that the content of the modified polyolefin is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and still more preferably 10 parts by mass or less.

In addition, the content of the amino-modified silicone with respect to the modified polyolefin was 1.0≤(mass of amino-modified silicone/mass of modified polyolefin)≤3.0. From this result, it was found that (mass of amino-modified silicone/mass of modified polyolefin) is preferably 0.5 or more, more preferably 0.7 or more, still more preferably 0.9 or more, and furthermore preferably 1.0 or more. In addition, it was found that (mass of amino-modified silicone/mass of modified polyolefin) is preferably 30.0 or less, more preferably 10.0 or less, still more preferably 5.0 or less, and furthermore preferably 3.0 or less.

Particularly, when 10 parts by mass of the amino-modified silicone-1, 5 parts by mass of the modified polyolefin-1, 10 parts by mass of the amino-modified silicone-2, and 5 parts by mass of the modified polyolefin-2 were included, the static friction coefficient was able to effectively maintain a low value.

In addition, the acid value of the modified polyolefin was 12 or more. Thus, the acid value of the modified polyolefin is preferably more than 1 (mgKOH/g) and 100.0 (mgKOH/g) or less, more preferably 5 (mgKOH/g) or more and 80 (mgKOH/g) or less, and still more preferably 10 (mgKOH/g) or more and 60 (mgKOH/g) or less.

As described above, in the grease composition according to the present embodiment, the fluidity of the grease composition is maintained by the silicone oil of the base oil. In addition, the amino-modified silicone and the modified polyolefin as additives are bonded to form a strong oil film, and therefore when the grease composition is used for a ball joint, oil film breakage can be prevented during the stopping of the ball joint. This can reduce the starting torque of the ball joint under a low temperature while reducing the static frictional force of the grease composition.

INDUSTRIAL APPLICABILITY

Using the grease composition of the present invention as a lubricant of a ball joint can reduce the starting torque and the operating torque, and the low temperature property can be improved. Using the ball joint using the grease composition of the present invention for suspension or steering of an automobile can improve steering stability or steering properties of the automobile.

The present application is based on Japanese Patent Application No. 2019-162868 filed on Sep. 6, 2019. All the contents are included here. 

1. A grease composition for a ball joint, comprising: a silicone oil as a base oil; an additive; and an amino-modified silicone and a modified polyolefin as the additive.
 2. The grease composition for a ball joint according to claim 1, comprising at least one of dimethyl silicone oil and phenyl-modified silicone oil as the silicone oil.
 3. The grease composition for a ball joint according to claim 1, wherein the amino-modified silicone is represented by a following general formula (1) or (2):

Organic group is —RNH₂, R represents an alkyl group, and n represents number of repetitions.

Organic group is selected from any

R and R′ represent an alkyl group, and m and n represent number of repetitions.
 4. The grease composition for a ball joint according to claim 1, comprising at least one of oxidized polyethylene, oxidized polypropylene, acid-modified polyethylene, and acid-modified polypropylene, as the modified polyolefin.
 5. The grease composition for a ball joint according to claim 1, wherein a content of the amino-modified silicone is more than 0 parts by mass and 30 parts by mass or less.
 6. The grease composition for a ball joint according to claim 1, wherein a content of the modified polyolefin is more than 0.1 parts by mass and 20 parts by mass or less.
 7. The grease composition for a ball joint according to claim 1, wherein a content of the amino-modified silicone with respect to the modified polyolefin is in a range of a following formula (3): 0.5≤(mass of amino-modified silicone/mass of modified polyolefin)≤30.0  (3).
 8. The grease composition for a ball joint according to claim 1, wherein an acid value of the modified polyolefin is more than 0.1 (mgKOH/g) and 100.0 (mgKOH/g) or less.
 9. The grease composition for a ball joint according to claim 1, wherein a static friction coefficient of the grease composition is 0.050 or less.
 10. The grease composition for a ball joint according to claim 1, comprising at least one of hindered phenol, alkylated diphenylamine, or a hindered amine light stabilizer as an antioxidant. 